1. Field of the Invention
This invention relates to electrical apparatus for detecting an overcurrent condition in a DC motor caused by the action of a DC motor controller which causes varying DC currents to flow in the power lines. These powr line currents reflect the magnitude and the duration of the motor loading.
2. Description of the Prior Art
In the operation of a DC motor, which in turn is operated by a controller and in conjunction with a load, the DC currents which flow in the power lines must be monitored in order to reflect the magnitude and the duration of the motor loading to prevent an overload condition. A standard procedure involves the use of an ammeter shunt placed in one of the controller power lines which produces a millivolt signal proportional to the magnitude of the established currents in the power lines. This proportional signal is then used to indicate the amount of motor loading. Various devices and modifications to a controller system has been employed in the prior art in order to attempt to provide an inexpensive and efficient overload monitoring device which can cut off power to the motor when the load demands become excessive.
One of the previous attempts to provide a over current protection for a montor driven device is indicated in the U.S. Pat. No. 4,160,282 to Dolinar et al. Basically this particular embodiment discloses a series connection of a shunt to a power source for conveying direct current power to an electrically powered vehicle. The shunt generates a voltage input signal proportional to the current drawn by the load and a solid state comparator receives the input signal and compares it to a reference signal having a magnitude representing the maximum level of current permitted to flow through the conductor. One of the immediate problems with the system of this type is that the relay 34 and its contact 32 is tripped whenever the current is in excess of a reference set value no matter for how long a period of time. Thus there is no provision or method of adjusting the system for taking into account short periods of slight overload which may be caused by transient operating conditions and thus the entire system would be closed down with no provision for restart except by a manual resetting of the relay device.
One type of control system which has been developed which overcomes the above-mentioned problems with regard to providing a cut off which allows for slight amounts of overload for short periods of time is addressed in the U.S. Pat. No. 3,558,978 to Nye. This circuit presents an electronic circuit breaker which has provisions for gradual and instantaneous cut off for a motor. An input circuit is connected to receive a first signal which is related to the level of the current flow through the motor and to combine this signal with the reference to produce a resultant input signal indicating the amount of which the motor current exceeds a predetermined reference level. An amplifier circuit using amplifier 36 is connected to provide an output control signal which varies as a result of the input signal changes. A feedback circuit including capacitor 40 and resistors 41 and 42 is coupled to an amplifier circuit to provide a proportional-integral relationship between the amplifier output signal and the input signal. At this point another circuit is used to couple the amplifier circuit and to disable the firing control circuit and interrupt the energization of the motor either immediately when a sudden motor overload exceeds a previously established level or after a delay period when a motor overload is less than the established level. One of the problems associated with this type of gradual and instantaneous cut off is that the feedback control circuit establishes both the levels for the instantaneous cut off and for the gradual cut off and that a change in a capacitor or a resistor built into the circuit at 40 or 41 and 42 is necessary in order to change the delay factor or the time at which a instantaneous cut off occurs. Another problem associated with a control circuit of this type is the amount of modification necessary to a particular motor load environment in order for the circuit breaker to function. Thus, in this particular instance a normal ammeter shunt is not satisfactory and further it is not possible to easily obtain a change in the point at which the firing circuit is disabled. That is, as previously mentioned, the instantaneous cut off and the gradual cut off are interrelated and cannot be adjusted without modifying several elements including the capacitor 40 and the resistor combination 41 and 42. Another area where the Nye reference presents problems is the necessity of a manual reset for each overload detection, as indicated by the reset 82.
Accordingly it is desireable to provide an overload protection device which overcomes the problems of the prior art by being able to use a signal from a standard ammeter shunt placed to measure DC motor currents for any horsepower motor for use in a constant potential contactor type or adjustable voltage type, with overload relay isolating supply, control system having a instanteous relay tripping function and a time dependent relay tripping function each of which are independent of each other and individually adjustable but are accomplished with solid state components integrated in a single device.